The invention concerns a device for coating a web of material traveling around a backing roller, with a flexible doctor having a foot secured in a clamping beam and a point supported by a supporting strip and with pressure-adjusting mechanisms positioned above the width of the doctor and acting independently of each other on individual points on the doctor. Devices of this kind are employed in particular to coat paper and cardboard.
They have a coating-application system with rollers or nozzles that apply a surplus of coating to the web. Downstream of the coating-application system is a doctor that reduces the coating to the desired thickness. The quality of the coating is decisively affected by the geometry of the area of the doctor that rests against the backing roller. The coating density is established by the pressure of the doctor against the backing roller, which is dictated by the tension on the doctor. To prevent sacrificing coating quality, one version of the method of applying pressure demands that the geometry of the point of the doctor be kept essentially constant.
When webs of paper or cardboard are coated, production-dictated fluctuations in the transverse cross-section of the web occur and make it necessary to vary the pressure of the doctor locally at various points along the operating width in order to obtain a uniform coating. The supporting strip in the generic device disclosed in German Pat. No. 2 825 907 is to a certain extent flexible and can be adjusted for this purpose by tension and compression screws along its line of support depending on whether the coating density is too high or too low at the point in question.
This known coating device has two serious drawbacks. First, local correction of the coating density along the operating width by means of the pressure of the doctor is impossible without simultaneously varying the geometry of the point of the doctor. This can result in local and temporary variations in quality and in a delay in the initiation of the desired effect. Second, it is impossible to automatically control the local distribution of pressure while the web is being coated.